The goals of the proposed research are: (1) To develop theoretical models to assess the importance of concentration polarization phenomena in ultrafiltering capillaries. These studies will require solutions to the coupled convection-diffusion and momentum equations which incorporate permeable walls and moving erythrocytes. Such calculations are needed to substantiate the validity of using our previously developed mathematical models for the calculation of effective ultrafiltration coefficients, and to guide us in determining the critical measurements which must be made to gain a better understanding of capillary hemodynamics and transcapillary water and solute exchange. (2) To characterize the permselectivity of the glomerular membrane in vivo using radioactively labeled macromolecules and micropuncture techniques. Gel filtration chromatography will be used to prepare narrow molecular weight fractions of certain macromolecules (polyethyleneglycol, polyvinylpyrrolidone, and dextran) for infusion into Wistar rats. By using solute containing fluid samples collected directly from glomerular capillaries and Bowman's space, we plan to characterize the glomerular membrane solute transport properties employing the theoretical concepts of non-equilibrium thermodynamics. (3) To measure, in vivo, capillary elasticity, hydraulic permeability and red cell velocities in the mammalian renal microcirculation. This will be done using electronic pattern recognition techniques, television microscopy, micropuncture, and ultramicroanalytical chemical assay methods. Such measurements will provide us with a basis for further modeling studies with a level of microscopic detail not yet possible with available data.